Internal combustion engines such as diesel engines and combustion engines such as boilers utilize fossil fuels. Combustion of fossil fuels is accompanied by generation of exhaust gas that contains NOX, SOX, oil, microparticles and the like, and which constitutes a source of environmental pollution. Accordingly, approaches adopted to reduce those sources of environmental pollution in exhaust gas include, for instance, improving fuels, enhancing combustion efficiency, and introducing exhaust gas treating equipment.
Improvements have been achieved in vehicles and overland facilities such as factories and thermal power plants.
However, engine emissions in marine or river vessels are far greater than those of automobiles or the like, while such vessels have virtually no surplus space amenable for installation of exhaust gas treating equipment. Moreover, adopting technologies and equipment that are geared toward overland uses into in vessels has proved difficult thus far from an economic standpoint.
For example, ammonia SCR (Selective Catalytic Reduction) is one instance of technologies for treating NOX in exhaust gas. Ammonia SCR is a method, implemented on land, that involves causing NOX in exhaust gas to react with ammonia, to trigger decomposition into water and nitrogen. However, ammonia SCR requires securing large amounts of ammonia, and has therefore proved difficult to utilize in vessels.
For instance, magnesium hydroxide methods and lime-gypsum methods in which scrubbers are utilized are implemented overland to deal with SOX. Techniques developed to adapt the foregoing methods to vessels include treatment methods that involve introducing seawater into a scrubber, to cause SOX in exhaust gas to be absorbed by seawater, and oxidizing thereafter SOX into sulfate ions by aeration.
Patent literature 1 discloses the feature of performing pulsed corona discharge on exhaust gas of a marine diesel engine, followed by introduction of the exhaust gas into a scrubber, and spraying of the exhaust gas with seawater to remove thereby nitrogen oxide and sulfur oxide from the exhaust gas.
Patent literature 2 discloses the feature of subjecting exhaust gas to a purification treatment by electrolyzing salt water to generate an alkaline solution and an acidic solution, and bringing the exhaust gas to be treated into gas-liquid contact with the acidic solution, and thereafter into gas-liquid contact with the alkaline solution.
Patent literature 1: Japanese Patent Application Publication No. H08-10564
Patent literature 2: Japanese Patent Application Publication No. 2003-284919
However, the effect of reducing the amount of SOX and the like in the methods disclosed in Patent literature 1 and 2 is not found to be sufficient. Further, these methods required a large amount of seawater during the purification treatment of exhaust gas. This gave rise to the problem of large power consumption, incurred by pump operation, during uptake of seawater to be used in the purification treatment, and during discharge of scrubber wastewater back into the ocean. No external power supply can be received at sea, and hence power consumption must be met by relying on power generated within the vessel. The power consumption of such pumps should thus be desirably reduced as much as possible.
Further, scrubber wastewater contains NOX, SOX, oil, microparticles and the like that are present in exhaust gas, and hence discharge of scrubber wastewater without having undergone any treatment will tend to be ever harder on account of water quality regulations by the International Maritime Organization.